Costs and benefits of interlarval cannibalism in Harmonia axyridis

Cannibalism, which is rather common in ladybirds, has been usually studied at the individual level: benefits of cannibalism for cannibals were estimated. Our study was conducted at the group level: we evaluated the overall effect of interlarval cannibalism on a group of Harmonia axyridis larvae of the fourth instar deprived of food, including both cannibals and their victims. Experiments showed that the probability of pupation in larvae which were kept individually was significantly higher than in larvae kept in groups of five, other conditions being the same. The proportion of samples in which at least one of five individuals pupated among the larvae kept individually was also higher than among those kept in groups suggesting that the eventual benefit of cannibalism was outweighed by the negative impact of aggressive interlarval interactions. The mean and minimum survival time in samples where none of five larvae pupated were longer when larvae were kept individually than when larvae were kept in groups. However, the maximum survival time (the survival time of the last larva in a sample) increased when larvae were kept in groups, which was the only one benefit of cannibalism found in our study. Under natural conditions, the possible adaptive value of this effect is that in the absence of natural prey, longer larval survival time proportionally increases the possibility of finding a new prey patch thereby ensuring survival of the population.